Implement light node - Githubissues

This PR implements light node which only fetches headers and relies on full nodes for complete block validation. Supproting changes:

Block database allows to store headers
Fixed bug in block fetching when unsuccessful fetch means that block will never be fetched from another node.
Refactoring of internal APIs. view doesn't leak into network layer. Necessary for light node which doesn't have blockchain state

hexresearch / hschain